It is understood in oil and gas production that heating a downhole fluid stream can (a) lower fluid stream viscosity, (b) reduce tubing friction losses, (c) reduce wellhead pressure requirements, (d) reduce or otherwise eliminate the formation of emulsions, and (e) improve pump efficiency, which in turn, can reduce the energy required to deliver a fluid stream to the surface from downhole and can also reduce the load placed on lift system components. It is also known that maintaining the temperature of a fluid stream above the cloud point (the point at which paraffin, hydrates, bitumen, ashphaltines and other complex hydrocarbons precipitate out of the fluid) can eliminate the build-up of restrictive deposits inside a production tubing string that can restrict fluid flow and lower the production rate of a well.
Current techniques used to heat and improve the flowability of fluid streams include resistance heating cables, solid resistance heating elements, induction heaters, and steam or hot oil injection. These techniques often have poor heat transfer characteristics and can lead to significant amounts of energy being lost to the surrounding environment and to non-productive parts of the well.
For instance, with resistance heating cables, which are strapped to the production tubing string to provide heat to the fluid stream inside the tubing during production, a central problem is created because a significant part of the cable is exposed to the surrounding well bore environment. This results in a significant amount of heat energy being lost to the surrounding environment, where it is of little value. Another problem with resistance heating cable systems is that it is extremely difficult to make certain that the heating cable maintains an unbroken contact with the production tubing since gaps where there is no contact will appear at locations where the cable does not lie flat on the tubing. These air gaps significantly lower the efficiency of heat transfer between the cable and the tubing string. Yet another problem with common resistance heating cable systems is that a significant portion of the heat energy, which is delivered to the production tubing, is used to heat the tubing and not the fluid inside. Finally, since none of the heat provided by resistance cable systems is to the fluid below the pump intake, fluid viscosity through the pump is unchanged and there is no benefit to pump performance or efficiency.
Solid resistance heating elements have also been used at the bottom of a production tubing string in order to heat fluid that passes over and around the heating element. The main problem with this configuration is that they have poor heat transfer characteristics due to a lack of fluid flow through the center resulting in internal and surface element temperatures that are significantly higher. The main result is poor efficiency in the heat transfer process. In order to compensate for this poor efficiency, these types of tools must operate with significantly higher surface temperatures, which can lead to coke formation on the heated surfaces. This build-up of coke further limits heat transfer and exacerbates the problem. Finally these heating elements are exposed to the well annulus with no insulating shroud. This means that a significant portion of the heat energy that they provide is lost to the surrounding environment with limited results.
Existing products also found in the marketplace include induction heaters, which warm the production casing or tubing using induced current in order to warm the production fluid stream inside the well bore. The main problem with induction heaters is that the clearance between the powered induction coil and the casing or tubing must be very small in order to maintain minimum levels of energy efficiency. Since the induction coil in most designs is located in the path of the production fluid stream, they often add significantly to pressure losses in the fluid stream defeating their purpose. In addition, placing an electrical current inside any component of a producing well such as the tubing or casing will significantly increase the corrosion rate and may cause premature failure.
Additional products found in the marketplace include steam or hot fluid oil injection products and methods where heated fluid or steam is injected into the well from the surface in order to remove wax and paraffin build-up or to increase the temperature of the fluid contained in the well bore or reservoir. The main problem with steam or hot oil injection products is that significant levels of heat energy are lost in these processes to non-productive parts of the well such as the casing, annulus and portions of the earth in contact with the casing that are not a part of the reservoir. In addition, the surface infrastructure required for permanent steam injection takes considerable space on the surface making this application undesirable in most offshore applications and populated areas.
An apparatus is needed that can increase the temperature and better regulate and improve the flowability a fluid stream.